With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more prevalent. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
One of the advances in portable communication technology is the move toward the implementation of a low intermediate frequency (IF) transmitter and receiver and a direct conversion transmitter and receiver (DCR). A low IF receiver converts a radio frequency (RF) signal to an intermediate frequency that is lower than the IF of a conventional receiver. A direct conversion receiver downconverts a radio frequency (RF) received signal directly to baseband (DC) without first converting the RF signal to an intermediate frequency. One of the benefits of a direct conversion or low IF receiver is the elimination of costly filter components used in systems that employ an intermediate frequency conversion.
Another advance in portable communication technology is the move away from bipolar complementary metal oxide semiconductor (Bi-CMOS) technology and the move toward implementing receiver components completely in CMOS technology. Implementing the receiver components completely in CMOS technology reduces cost, power consumption and the physical space used on the device.
Unfortunately, implementing the receiver components using CMOS technology results in the increase in some noise parameters in the receiver, and particularly in the mixer. Regardless of the type of transceiver used in the system, one or more mixers are used to upconvert the transmit signal to an RF level and to downconvert the received RF signal. A mixer combines the RF signal with a reference signal, referred to as a “local oscillator,” or “LO” signal. The resultant signal is the input signal at a different, and, in the case of a downconverter, typically lower, frequency.
The noise performance of a mixer implemented in CMOS technology typically suffers due to so called “1/f” noise (also referred to as flicker noise ) in the mixer core at low frequency offsets. The spectral density of 1/f noise increases significantly as the frequency is reduced. For example, at a low intermediate frequency (IF) offset of 100 kHz, which is a typical frequency to which a received signal is downconverted in a low-IF or a direct conversion receiver, the 1/f noise significantly raises the noise figure (NF) in the receiver. In addition, the large LO and radio frequency (RF) transitions that are used to reduce the noise floor in the mixer contribute to what is referred to as LO self-mixing, where the LO signal is undesirably coupled into the desired receive signal and the combination of the LO signal and the receive signal is undesirably multiplied with the LO signal. The large LO and RF transitions also contribute to RF self-mixing, where the RF signal is undesirably coupled into the LO signal, which leads to a DC signal that is proportional to the RF signal, which further reduce the performance of the receiver. Further, any DC offset mismatch between transistors in the mixer leads to a poor second intercept point (IP2) performance.
Past systems have attempted to minimize the 1/f noise by using a low-IF architecture, implementing large LO signal transitions, or by implementing physically large transistors in the mixer to reduce the effect of the 1/f noise. Unfortunately, these solutions generally consume additional power, degrade linearity of the mixer, contribute to LO self-mixing due to high LO drive power required for the large transistors, and consume additional physical space.
Therefore, it would be desirable to reduce the noise contributed by a mixer in a receiver.